Figure 2. Key elements of the tropical Atlantic in situ observing system during a — and b — Squares show moored buoy locations, circles indicate additional measurements made from the moorings, and triangles show locations of deep Argo and biogeochemical Argo floats during September Tropical Atlantic variability TAV influences a wide range of societally important phenomena on different timescales that span the physical, biogeochemical, and ecological systems and their interactions. This section summarizes the key societal drivers in the tropical Atlantic. One of the most important climate- and weather-related societal drivers is continental rainfall.
The impacts of tropical Atlantic climate variability on rainfall are strongest in South America and West Africa. Its onset is typically in late June or early July and it ends normally from late September to October Sultan and Janicot, Knowledge of the development of the WAM is important for agricultural planning, as an unanticipated late onset and early demise of the monsoon can lead to crop failure.
There is stronger interannual variability at the Guinea Coast compared to the Sahel, and rainfall anomalies often extend zonally across West Africa Giannini et al. A recent example is the — severe Sahel drought and famine, which was caused by below-normal and erratic rainfall in and poor harvests in and 1. The Sahel region also experiences strong rainfall variability on decadal and multidecadal timescales.
There was a period of severe drought between the s and s that strongly impacted West African agriculture and economies. Rainfall in the Sahel has increased since the s, but has not recovered to pre-drought levels Nicholson et al. Through its impacts on soil moisture, vegetation, and albedo, the rainfall received during a given year affects the likelihood of drought the following year, acting to enhance decadal-multidecadal variations of Sahel rainfall Zeng et al.
The long-term trend of Sahel precipitation in response to the intensification of the hydrological cycle due to climate change is still unclear and may have significant consequences for West African populations Druyan, ; Monerie et al. The semiarid region of Northeast Brazil has also experienced strong rainfall variations and extreme droughts.
This region is highly susceptible to drought because its short March—May rainy season is dependent on seasonal sea surface temperature SST and rainfall patterns in the tropical Atlantic. During —, Northeast Brazil experienced its most severe and prolonged drought since Brito et al. Prior to the recent drought period, Northeast Brazil experienced a severe flood in that has been linked to anomalous SSTs in the tropical Atlantic Foltz et al. There is strong interannual and decadal variability of Northeast Brazil rainfall Nobre and Shukla, , along with a downward long-term trend of precipitation and increasing trend in air temperature Lacerda et al.
During the years —, the Amazon region experienced some of the most severe droughts and floods in its recorded history. The droughts occurred during the dry season June—September , when the ecosystem is particularly vulnerable to stressors. Normally the Amazon is a carbon sink, absorbing CO 2 from the atmosphere.
However, during — the Amazon was a net carbon source due to drought-stressed and dying trees, combined with increased occurrence of fires Zeng et al.
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At the other extreme, the Amazon floods of and were the largest going back several decades Satyamurty et al. It is unclear whether the recent increase in variability is part of a longer trend driven by climate change or due to natural variability. In summary, there is a strong societal need for accurate predictions of rainfall to improve agricultural productivity, allow for more efficient use of water resources, and protect homes and infrastructure against floods.
There is also a need to mitigate disease outbreaks, which commonly occur following large floods. Rainfall predictions are needed on many different climate timescales, ranging from intraseasonal to multidecadal. Climate change projections of rainfall and the occurrences of droughts, floods, and extremes in rainfall intensity are also a necessity for developing countries surrounding the tropical Atlantic Ocean.
Tropical cyclones TCs are one of the deadliest and most destructive hazards in the tropics and subtropics. Threats include storm surge, damaging winds, and inland flooding from rainfall. Developing countries and low-lying coastal areas are particularly vulnerable. Adjusted for inflation, 9 of the 10 costliest hurricanes have occurred since The increasing destruction is likely a result of coastal population growth in the United States as well as natural and human-induced changes in the large-scale hurricane environment that can influence TC intensity, rapid intensification RI , translation speed, and rainfall Goldenberg et al.
The Atlantic basin as a whole has experienced large variations in TC activity on interannual and longer timescales.
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The , , and Atlantic hurricane seasons were extremely active, with 28, 19, and 17 cyclones of at least tropical storm strength, respectively. The and seasons were the costliest on record at the time of occurrence. There has also been significant decadal-multidecadal variability of TC activity in the Atlantic, with above-normal activity during the s and 50s, below-normal from the 60s to the early 90s, and above-normal since the mids Goldenberg et al.
The magnitude of hurricane RI increase in maximum wind speed of at least 25 kt in 24 h has increased in the central and eastern tropical Atlantic since the s Balaguru et al. Whether a storm will undergo RI is particularly difficult to predict Kaplan et al. When RI occurs before landfall, destruction and loss of life can be catastrophic. Ultimately, in terms of seasonal prediction, what matters most for coastal residents and planning agencies is the number and severity of land-falling TCs. There have been marked changes in Atlantic land-falling TCs in the past several decades Wang et al.
However, the number of landfalling TCs is very difficult to predict. There are indications that TC activity and intensification before landfall may be increasing in the Atlantic due to global warming Emanuel, , ; Webster et al. In summary, there is a need for improved intraseasonal and seasonal predictions of TC activity, including landfalls, and more reliable decadal-multidecadal projections. There are also uncertainties related to how Atlantic TC activity will change in response to global warming. This is particularly important for highly populated low-lying coastal areas in the southeastern United States, which will likely become more susceptible to storm surge inundation as sea level rises.
Improved intraseasonal and seasonal predictions and longer-term projections will allow coastal communities to prepare and allocate resources for post-storm recovery. One of the important unknowns in the future global carbon budget is the extent to which the ocean sink keeps pace with anthropogenic CO 2 emissions.
The tropical Atlantic is the second largest source, after the tropical Pacific, of oceanic CO 2 to the atmosphere, releasing about 0. While uptake of anthropogenic CO 2 by the ocean regulates the atmospheric CO 2 concentration, it also leads to ocean acidification, with significant but poorly understood consequences for marine organisms and ecosystems Feely et al.http://precisionpagesmedia.com/2998.php
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Carbon trends remain unclear because of short records and high natural variability in the tropical Atlantic, though there are indications of significant decadal variations that have implications for anthropogenic CO 2 uptake Park and Wanninkhof, Oxygen minimum zones OMZs are found at intermediate depths — m in the eastern tropical oceans off the equator Karstensen et al.
The OMZs in the eastern tropical Atlantic are split into shallow m and deep m branches Monteiro et al. The shallow OMZs overlap with the euphotic zone and hence have a direct impact on ecosystems, carbon export, and the release of CO 2 and other climate-relevant trace gases like N 2 O to the atmosphere.
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The carbon and oxygen cycles and ecosystems in coastal regions are influenced by river outflow and upwelling. The rivers also deliver high loads of nutrients, which lead to high oceanic productivity near the river mouths. These systems are particularly vulnerable to ongoing warming, deoxygenation and acidification Gruber, It is important to understand what drives biological production within these regions in order to understand ecosystem dynamics and also to constrain the regional and global carbon cycles.
There are several important factors that control biological production in eastern upwelling regions, including along-shore winds, eddy activity, and mixed layer depth Lachkar and Gruber, Low oxygen conditions affect the carbon cycle, ecosystems and fisheries.
Trace elements e. The cycling of these micronutrients is thus critically linked to carbon cycling. Many coastal communities surrounding the tropical Atlantic Ocean rely on seafood for sustenance. The importance of fisheries in the tropical Atlantic can be demonstrated most easily by the total catch and dependence on the region.
Approximately 10 million tons of seafood from The fishing sector is also very important in tropical Atlantic coastal countries. There are 8 million fishers in Africa, Latin America, and the Caribbean, though not all of them are operating in the Atlantic; many foreign fleets Korean, Chinese, Russian, European also use tropical Atlantic resources. Threats to fisheries include overfishing, pollution, and invasive species. The frequency of marine heatwaves has increased significantly in the tropical North Atlantic during the past 35 years and is expected to increase further in response to global warming Oliver et al.
Marine heatwaves are defined as anomalously warm SST that lasts for five or more days, with temperatures warmer than the 90th percentile based on a year historical baseline period Hobday et al. They can have detrimental effects on marine organisms, including coral bleaching, disease outbreaks, and forced migration Comte and Olden, ; Hughes et al. Future responses of marine organisms to climate change and the implications for the biogeochemical cycles and fisheries are unclear. These events can have significant negative impacts on local economies and ecology Hu et al. It is unclear what has caused the increase in Sargassum in the western tropical North Atlantic and Caribbean.
Hypotheses include changes in upper-ocean temperature and nutrients or anomalous winds and ocean currents. Previous events such as the Deepwater Horizon drilling rig explosion can also have serious negative consequences for local ecosystems and economies and require knowledge of the ocean circulation.
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Transport and fishing vessels in the Atlantic are sources of marine pollution such as plastics, hydrocarbons, and particulate materials. Overall, there are relatively few records of pollutants that have emissions high enough to cause harmful consequences in the open ocean. Mercury and plastic pollution, nevertheless, show us that adverse effects of those pollutants in marine ecosystems can be widespread. Plastic is a pervasive pollutant of high concern. Its use has increased 20 times in the past 50 years and is expected to double in the next 20 years 6.